An exhaust gas purifier, shown in FIG. 11, has been disclosed in Japanese Patent Laid-Open No. Hei 3-22913, wherein a perforated pipe 02 having a plurality of ventilation holes 03 spaced apart from each other at intervals is disposed along the inner peripheral surface of an expansion chamber 01 connected to an exhaust port of an internal combustion engine.
In the purifier shown in FIG. 11, a tail pipe 04 directed toward the upstream side is disposed in the expansion chamber 01 at the downstream end thereof in such a manner as to be coaxial therewith. An exhaust gas passable catalyst 05 is mounted at the upstream end of the tail pipe 04. Part of the exhaust gas flowing from the exhaust port of the internal combustion engine into the expansion chamber 01 is brought into contact with a catalyst supported on the surface of the perforated pipe 03 positioned on the upstream side, to be thus purified; while the exhaust gas in the expansion chamber 01 is brought in contact with the exhaust gas passable catalyst 05 on the downstream side, to be thus purified.
The purifier shown in FIG. 11, however, has the following disadvantage. In such a purifier, the gas flowing nearer the inner peripheral surface of the expansion chamber 01, is significantly reduced in flow rate because presented by a resistance of the inner peripheral surface of the expansion chamber 01. As a result, most of the exhaust gas passing through the center of the expansion chamber 01 at a high flow rate tends not to be brought in contact with the catalyst supported on the perforated pipe 03 adjacent to the inner peripheral surface of the expansion chamber 01, and to be discharged from the tail pipe 04 to the exterior, thus failing to obtain a sufficient purifying effect by the catalyst supported on the pipe 03.
In this purifier, the exhaust gas passable catalyst 05 is disposed at the center of the downstream side of the expansion chamber 01 in such a manner as to project from the downstream side to the upstream side. Consequently, even if the size and shape of the expansion chamber 01 is set to enhance the scavenging efficiency by the inertia effect of an exhaust gas in a normal operational range of the internal combustion engine, as well as the blow-off preventive effect using a reflection wave of the exhaust gas, these effects cannot be achieved by the presence of the above exhaust gas passable catalyst 05, thus failing to enhancing the scavenging efficiency.
An exhaust gas purifier shown in FIG. 12 has been disclosed in Japanese Patent Laid-Open No. Hei 3-127017. This is modified from the purifier shown in FIG. 11. In this purifier, an upstream end 011 of an expansion pipe 010 formed in a shape similar to that of a prior art expansion chamber is connected to an exhaust port of a two-cycle internal combustion engine mounted on a motor-bicycle (not shown). The downstream end 012 (on the right side in the figure) of the expansion pipe 010 is closed. An exhaust pipe 013 is branched from the expansion pipe 010 at a position near the upstream end 011, and a thin-wall catalyst element 014 is disposed on the inner peripheral surface of the exhaust pipe 013 at a position near the upstream end. In the catalyst element 014, catalyst is supported on the surface just as in the perforated pipe 03 of the purifier shown in FIG. 11. The downstream end of the exhaust pipe 013 is connected to a silencer 015.
In the purifier shown in FIG. 12, the scavenging efficiency is improved because any component disturbing the reflection of a pressure wave is not present on the inner surface of the expansion pipe 010. However, like the catalyst supported on the pipe 03 shown in FIG. 11, exhaust gas cannot be sufficiently brought in contact with the catalyst element 014, thus failing to obtain a high scavenging efficiency.